1. Field of the Invention
The present invention relates generally to magnetic disk drives (disk drives), and more particularly to an efficient method of using a servo track writer (STW) for continuously recording servo information along one, continuous, single helix spiral.
2. Description of the Related Art
A conventional disk drive has a head disk assembly (HDA) including at a least one disk, a spindle motor for rapidly rotating the disk, and a head stack assembly (HSA) that includes an actuator assembly and a head gimbal assembly (HGA) with a transducer head for reading and writing data. The HSA is part of a servo control system that positions the transducer head over a particular track on the disk to read or write information from that track. The HSA earns its name from the fact that it generally includes a plurality of HGAs that collectively provide a vertical stack of heads called a xe2x80x9chead stack.xe2x80x9d
The industry presently prefers a xe2x80x9crotaryxe2x80x9d or xe2x80x9cswing-typexe2x80x9d actuator assembly that conventionally comprises an actuator body which rotates on a pivot assembly between limited positions, a coil that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor (VCM), and an actuator arm that extends from the opposite side of the actuator body to support the HGA.
A disk drive is ultimately used to store user data in one or more xe2x80x9cdata tracksxe2x80x9d on the surface of its disks. Such data tracks are most commonly arranged as a plurality of concentric data tracks, but some disk drives have had a spiral data track as shown, for example, in U.S. Pat. Nos. 4,636,885; 5,583,712; and 5,619,387. In either case, special servo information is recorded on at least one disk surface so that the disk drive""s servo control system may control the actuator, via the VCM, to accurately position the transducer head to read or write user data to or from the data tracks. In colloquial terms, the servo information provides the servo control system with the xe2x80x9cyour head is herexe2x80x9d data it needs to attain and then maintain a desired head position. In operation, the disk drive""s servo control system intermittently or continuously processes (read only) the pre-recorded servo information just before or while the disk drive processes (reads or writes) user data in the data tracks.
The servo information is factory recorded at the time of manufacture using an expensive and low-throughput manufacturing fixture called a servo track writer (STW). The STW records the servo information in special xe2x80x9cservo tracksxe2x80x9d on each surface of each disk, or on one dedicated disk, for later use by the servo control system when the drive is in the hands of the user. The servo tracks are generally used throughout the life of the disk drive without modification.
Earlier disk early drives used a xe2x80x9cdedicated servoxe2x80x9d system where one head and one disk surface provide the servo information for all of the other heads and disk surfaces. As shown in FIG. 2, however, the industry presently prefers an xe2x80x9cembedded servoxe2x80x9d system wherein the servo information is interspersed amongst the data on each surface of each disk. The servo information is contained in servo wedges 300 that are each divided into a plurality of servo sectors 310. The servo sectors 310 are recorded concentrically in order to provide numerous concentric servo tracks (one entire rotation of servo sectors 310). The servo wedges 300 precede a corresponding number of data wedges 400 that are ultimately used to record concentric data tracks 10 that are divided into a plurality of data sectors (not shown). Each data wedge 400 may contain a whole or fractional part of one or more data sectors (not shown). Because the servo information is provided in servo sectors 310, an embedded servo system is sometimes called a xe2x80x9csector servoxe2x80x9d system.
In recording the embedded servo information, the STW takes temporary control of the drive""s write operation, repeatedly locates the write transducer to a desired radial position, and then writes, erases, or does nothing (remains idle) at specific angular positions between the head and a reference position of the disk as the disk rotates beneath the write transducer. In order to precisely locate the head where needed, a conventional HDA has first and second access ports (later covered with adhesive labels) for allowing the STW to xe2x80x9creach inxe2x80x9d and temporarily control the radial position of the actuator and measure the angular position of the disk while recording the servo information. As to the radial position of the actuator, the conventional STW inserts a moveable xe2x80x9cpush pinxe2x80x9d into the first port, commands the HDA""s VCM to bias the actuator against the push pin, moves the push pin against the bias to move the actuator and the attached headstack, and measures the position of the push pin with a laser interferometer to control the radial position of the write transducer carried by the pin-guided actuator. As to the angular position of the write transducer relative to an index position of the disk, the conventional STW inserts a stationary xe2x80x9cclock headxe2x80x9d into the second port, records a xe2x80x9cclock trackxe2x80x9d containing thousands of xe2x80x9cclock marksxe2x80x9d and one xe2x80x9cindex markxe2x80x9d (e.g. an extra clock mark or a gap) on a disk surface of one of the disks, and measures the angular position of the write transducer relative to the index mark by detecting the index mark and thereafter tracking (i.e. counting) the intermediate clock marks.
The conventional STW puts an embedded servo pattern onto a disk by recording concentric servo tracks in a plurality of discrete concentric xe2x80x9cpasses.xe2x80x9d Each pass consists of moving the push-pin to xe2x80x9cstepxe2x80x9d the headstack to a desired radial position, allowing the head to xe2x80x9csettle,xe2x80x9d and during one ensuing revolution of the disk, writing new servo information, erasing overlapping portions of previously written servo information, or remaining idle (neither writing nor erasing). On the first pass, the STW moves the write transducer to an outer diameter of the disk, and then records magnetic transitions at discrete angular intervals to record the servo information including track identification (track ID) data and servo bursts. During the second and each of the thousands of subsequent passes, the STW steps the write transducer inward by a fraction of a data track pitch (e.g. xc2xd), waits for the write transducer to settle (as much as one full revolution), and then records the servo information during another full revolution, writing more magnetic transitions, trimming overlapping portions of previously recorded transitions, or holding idle, as appropriate for the desired servo pattern. In order to record concentric servo tracks, therefore, the STW must repeatedly step, wait, and record.
The conventional method of recording concentric servo tracks creates a manufacturing bottleneck because each HDA must remain in the STW for an extensive amount of time in order to step, wait, and record each pass that collectively make up the required servo information. It takes a relatively long time to make thousands of passes of step, wait, and record. For example, given a disk drive that has a spindle motor that rotates at 4,500 RPM, an actuator with an effective stroke (ES) of one inch, and an intended data track density of 8,000 tracks per inch (TPI), and further assuming that the STW steps the write transducer by xc2xd a data track pitch per pass, it would take 3.56 minutes to record the servo information, i.e.:                     8,000            ⁢              xe2x80x83            ⁢      TPI      *      1      ⁢              xe2x80x83            ⁢      ES              1      ⁢              /            ⁢      2      ⁢              xe2x80x83            ⁢      TP      *      4500      ⁢              xe2x80x83            ⁢      RPM        =      3.56    ⁢          xe2x80x83        ⁢    minutes  
The 3.56 minutes assumes 100% STW efficiency. In reality, however, there are other overhead times associated with recording the servo information. As noted above, the write transducer must be stepped after ending one pass and before starting another. The pass to pass xe2x80x9cseek and settlexe2x80x9d time, or time lag before the write transducer is ready to start a new pass, is one of the more costly overhead times associated with writing concentric servo tracks. In the above-referenced disk drive, for example, the cumulative effect of a seek and settle latency of one revolution per pass would add another full 3.56 minutes to the time it takes to record the servo information on one disk surface. Assuming that the overhead time related to pre-servowrite procedures like drive load, drive spin-up, clock closure, and find edge (push pin/actuator closure and move to start position) and post-servowrite procedures like spin-down and drive unload combine to 0.88 minutes (53 sec.), the total time required to xe2x80x9cbank writexe2x80x9d or xe2x80x9cgang-recordxe2x80x9d the servo information on all disk services, is as follows:
3.56 min.+3.56 min.+0.88 min.=8.00 minutes
Significantly, the seek and settle latency between steps comprises a large portion of the STW time. In the above disk drive, for example, by just eliminating the seek and settle latency we would reduce the STW cycle time by 44.5%:             3.56      ⁢              xe2x80x83            ⁢      min              8.00      ⁢              xe2x80x83            ⁢      min        =      44.5    ⁢    %  
Perhaps a spiral servo track would work. Others have disclosed the use of a spiral servo track instead of concentric servo tracks as shown, for example, in the ""885 and ""712 patents. The latter patent, in fact, suggests using a spiral servo track (called a xe2x80x9cservo spiralxe2x80x9d) to track follow along a circular data track (see FIG. 4A). The 855 and ""712 patents, however, do not teach or suggest any particular method of actually recording the spiral servo track while the drive is in the STW.
The disk drive market is extremely competitive and drive makers are continually striving for efficiencies in order to remain profitable. STW""s are very expensive devices (upward of $100,000) and it takes a long time to servowrite each disk drive (several minutes per drive). Achieving efficiencies in the servowriting process, therefore, may significantly reduce the overall cost of manufacturing disk drives. Consequently, there remains a need for a method of recording servo information which eliminates the seek and settle times associated with the prior art method of recording concentric servo tracks, and thereby minimizes the required STW time for each drive.
In a first aspect, the invention may be regarded as a method of continuously recording servo information on a magnetic disk with a write transducer that is guided by a servo track writer, the method comprising the steps of: defining a continuous spiral path; continuously guiding the write transducer over the magnetic disk to follow the continuous spiral path and while the write transducer is being guided along the continuous spiral path: recording a first plurality of radially partial servo data portions on the magnetic disk during a first complete revolution of the magnetic disk; and recording a second plurality of radially partial servo data portions during a second complete revolution on the magnetic disk to form at least part of a plurality of radially complete servo data sectors. In a preferred embodiment, the write transducer is guided along a continuous spiral path having a radial pitch that is less than a width of the write transducer and the second plurality of radially partial servo data portions overlap the first plurality of radially partial servo data portions.
In a second aspect, the invention may be regarded as a method of continuously recording servo information within a servo sector on a magnetic disk with a write transducer that is guided by a servo track writer, the method comprising the steps of: moving the write transducer over the magnetic disk along one continuous, single-helix spiral stroke; and recording the servo information in the servo sector while the write transducer is moving along the one continuous, single-helix spiral path.
In a third aspect, the invention may be regarded as a method of using a servo track writer to move a write transducer and record servo information on a magnetic disk, the method comprising the steps of: rotating the magnetic disk; moving the write transducer in a radial direction at a constant radial velocity to traverse a spiral path relative to the rotating magnetic disk; and recording servo information on the magnetic disk within a spiral stroke while the write transducer is moving along the spiral path and with a stroke width substantially equal to one width of the write transducer, the constant radial velocity being less than one width of the write transducer per revolution of the rotating magnetic disk such that each revolution of the spiral stroke is overlapped by a subsequent revolution of the spiral stroke.